The configuration, for example, shown in FIG. 5 has been used in an apparatus for reproducing magnetically recorded data using magneto-resistive heads.
Specifically, in FIG. 5 data recorded on, for example, magnetic tape 51 are reproduced by a magnetoresistive head (hereinafter referred to as MR head) 52. The reproduction signal is extracted to the outside of, for example, a rotating drum (not shown in the figure) through a rotary transformer 53 and then supplied to a reproduction amplifier 54.
Hereupon, a low-frequency component of the extracted signal has been cut off through the rotary transformer 53, and a low-frequency component correction circuit 55 is provided on the output side of the reproduction amplifier 54 to correct the low-frequency component cut off. The signal from the low-frequency component correction circuit 55 is supplied to an integrating equalizer 56 having characteristics approximately reverse to the characteristics of magnetic recording and reproduction. Further, the signal from the integrating equalizer 56 is supplied to a phase equalizer 57 which compensates in the magnetic recording the phase rotation due to the orientation of the medium such as tape.
Then, the signal from the phase equalizer 57 is supplied to an analog cosine equalizer 58 which is to absorb changes in frequency characteristics based on a state of the magnetic recording and reproduction system. Accordingly, through the operation of these three equalizers, the magnetic recording and reproduction system can realize almost reverse characteristics, thereby enabling a signal equivalent to that at the recording to be extracted. This signal is then extracted through an automatic gain control amplifier (hereinafter referred to as an AGC amplifier) 59 for the purpose of removing amplitude variation.
The signal from the AGC amplifier 59 is further supplied to an A/D converter 60. Also, the signal from the AGC amplifier 59 is supplied to a digital PLL circuit 61 to extract a clock signal contained in the supplied signal. Then, the extracted clock signal is supplied to the A/D converter 60 and the signal from the AGC amplifier 59 is digitized. The digitized signal is then supplied to a decoder 62 using, for example, a Viterbi algorithm (Viterbi decoder), and the decoded signal is obtained at an output terminal 63.
In this manner the reproduction of data recorded on, for example, the magnetic tape 51 is performed using the MR head 52. When, however, this type of MR head is used to reproduce data recorded on, for example, magnetic tape, a problem of the so-called thermal asperity noise (hereinafter referred to as TA noise) occurs. Namely, TA noise is a specific problem when using the MR head to reproduce recorded data.
Hereupon, the MR head is a device which detects changes in resistance values of an element due to the rotation of the magnetization angle inside the magneto resistive element caused by an external magnetic field. In this case, the external magnetic field represents a magnetic field generated by the magnetization pattern recorded on tape or a disk. Also, in the MR head this change in resistance is obtained as the change in voltage. Therefore, based on Ohm's law voltage of the output signal from the MR head is equal to the value in which a measured current is multiplied by the resistance change as expressed [σV=IσR].
However, when the MR head is collided with projecting material such as dust occurred on media, for example, tape, disk or the like, such a phenomenon occurs that resistance values suddenly (in 1 μsec. or less) change due to frictional heat generated by the collision and then slowly return (in a few μsec.) due to thermal diffusion. In other words, the resistance value of the magnetoresistive element is also changed by such heat, and fluctuations in the resistance value caused by the heat are greater than changes due to the above-mentioned external magnetic field.
When fluctuations in the resistance value due to the heat occur, changes will also occur in the voltage of output signal from the MR head in which the above described resistance change is obtained as changes in the voltage. Specifically, as shown on the left side of FIG. 6, for example, the reproduction signal held at a constant amplitude by the AGC amplifier 59 suddenly changes immediately after a collision and then slowly changes as returning. This change is called thermal asperity (TA), and noise caused by TA is called TA noise.
Moreover, experimentation verified that the above TA noise caused effects as described below. Specifically, FIG. 7 shows the configuration of an apparatus used for the experimentation, in which a signal obtained by convoluting, for example, a Lorentz 7th order M-sequence signal shown at the lower left of the figure with pseudo TA noise as shown above is used as an input signal. Such an input signal is supplied to an AGC amplifier 704 through an integrating equalizer 701, a phase equalizer 702 and a cosine equalizer 703.
In this experimental circuit, when the input signals shown in FIGS. 8B, 9B and 10B are each supplied to the integrating equalizer 701, the output signals shown in FIGS. 8A, 9A and 10A are obtained at the AGC amplifier 704, respectively. FIGS. 8A and 8B show an example in which the lower frequency component of the input signal is not cut off, and therefore in this case, TA noise of, for example, approximately 5 μsec. has a lasting effect for about 70 to 80 μsec.
Compared with this, when the lower frequency component of the input signal is cut off at, for example, 285 kHz, the effect of TA noise lasts 30 to 40 μsec. as shown in FIG. 9. Further, when the lower frequency component of the input signal is cut off at 595 kHz, the effect of TA noise lasts 10 to 20 μsec. as shown in FIG. 10. Consequently, it was verified experimentally from these results that by cutting off, for example, the lower frequency component of the input signal the length of the TA noise effect is shortened.
Through these experiments, the cause of the above-mentioned lasting TA noise effect is judged to be the fact that the dynamic range of the internal circuit is saturated with respect to an input signal having a large DC component such as TA noise, thereby generating a surge in the output signal from the AGC amplifier 704. As a result, when such a surge occurs, the error rate of the reproduction signal will increase and therefore problems such as picture freezing, audio discontinuity or others will occur in equipment such as a digital VTR.